The present invention relates to a semiconductor power module, and more particularly, to a semiconductor power module having a base substrate, a ceramic insulating substrate, and a semiconductor chip which are connected with solder.
Generally, a semiconductor power module is configured so that an Si chip, an insulating substrate of AlN, Al2O3 or the like, a Cu base substrate or the like excellent in thermal conductivity are soldered, and filled with silicon gel.
For an IGBT module excellent in thermal conductivity, an AlN insulating substrate-Cu base substrate is used. As solder for post-fitting of the AlN insulating substrate and the Cu base substrate, Pb-60Sn eutectic solder has been used so far.
Even if large warpage occurs to a large-sized substrate immediately after soldering, the warpage is relieved by creep peculiar to the Pb-60Sn eutectic solder, and the warpage is reduced with lapse of time. In the outermost peripheral portion of the solder, the solder structure follows deformation of the substrate and stress, and a crack does not occur easily. Therefore, there has been no problem as the life.
Meanwhile, elimination of Pb is demanded due to the environmental problem, and in Japan, Sn-3Ag-0.5Cu is used in a wide range in packaging of household appliances, communication, computers and the like.
However, the power module requires soldering of a substrate with a large area, and it is found out that with Sn-3Ag-0.5Cu which hardly causes creep deformation with respect to large deformation followed by warpage of a substrate, reduction in life remarkably occurs with respect to the thermal cycle and TFT tests. Therefore, the conventional module structure is in the situation where it has difficulty in securing reliability.
Concerning a Sn—Ag—Cu—In solder composition component, U.S. Pat. No. 5,520,752 is known.
On the other hand, in the AlN insulating substrate and Cu base substrate configuration, elongation of the life of the power cycle is the longtime problem. Namely, breaking of wire due to peeling of an Al wire bonding portion occurs, and this limits the life of the entire module.
Because of consideration to the environment, use of Pb-free solder becomes essential, and Sn-3Ag-0.5Cu solder which is generally used needs to be successfully used for a power module. The disadvantage of the Sn-3Ag-0.5Cu solder which has creep resistance but cannot perform creep deformation needs to be overcome to ensure the life.
Thus, by coating an epoxy resin (hereinafter, sometimes abbreviated simply as a resin hereinafter) with adhesion having suitable physical properties onto the periphery of a solder outermost peripheral portion which is the start point of occurrence of a crack, the breakage mechanism is changed, crack occurrence of solder is inhibited and delayed, and crack developing speed can be made low. In order to increase the life of the AlN insulating substrate and the Cu base substrate, a resin having a Young's modulus capable of restraining both of them in terms of stress and having a linear expansion coefficient close to solder, Cu, AlN and the like is coated to the end portion. Thereby, the thermal cycle life of Sn-3Ag-0.5Cu solder can be significantly increased.
The structure which covers the entire power module with an epoxy resin and secures high reliability in the temperature cycle test and the like is already shown (For example, see JP-A-2004-165281).
The problem in the structure which seals the entire power module with an epoxy resin is the problem of warpage of the module when increased in size.
Namely, in the module, warpage occurs, which is influenced by the quantitative ratio related to the thick Cu base substrate with high flexural rigidity and physical properties of the epoxy resin. First, in the warpage which occurs when the Cu-base substrate and the ceramic substrate are soldered, residual stress exists in the early stage, and the warpage relates to yield reliability of the soldering process step. In addition to this, the warpage relating to the resin physical properties and resin thickness further occurs. Each warpage needs to be suppressed in a certain range.
In this respect, in the method for coating only the solder peripheral portion with a resin, warpage does not relate to rigidity of the resin coating thickness. Therefore, the resin coated on the periphery has less influence on warpage, and this method has less problem as compared with the structure which resin-molds the entire module. On the other hand, this method increases stress load on the peripheral resin portion. The resin has the main purpose of providing the effect of direct reinforcement of solder by coating the solder portion at the outermost peripheral portion where stress concentrates remarkably. This is the structure in which solder and the resin at the peripheral portion undertake stress and strain that occur due to thermal expansion difference between the Cu base substrate and the ceramic substrate since the resin does not exist on the module central portion.
On the other hand, in order to increase the life of a power cycle, it is necessary to increase the life of the Al wire bond part. In order to increase the life of the wire bond (hereinafter, abbreviated as WB) part, it is necessary to prevent thermal fatigue of the WB part by ON-OFF action, or to delay thermal fatigue deterioration. Long-time use increases the temperature of the perimeter of the WB part little by little, and thereby, accelerates fatigue deterioration of the WB part.
Thus, the first object of the present invention is to provide a semiconductor power module with favorable reliability, which prevents acceleration of fatigue deterioration by reducing rise in temperature of the wire bond part, and significantly increases solder life.
It is found out that since Sn-3Ag-0.5 Cu solder is not capable of creep deformation though it has creep resistance, it cannot relieve occurrence of warpage of a large-sized substrate of a power module or the like by creep, and since as a result, high stress always acts on the solder, the life of the solder becomes short contrary to expectations. Therefore, the thermal cycle test with the configuration with a very low strain rate was conducted with the model equivalent to a real thing with large heat capacity, the life was evaluated in each promising Sn solder having realizability, and selection of the solder composition was performed.
A second object of the present invention is to provide a semiconductor power module designed to increase the life of the solder used between a ceramic substrate and a base substrate against the temperature change of intermittent energization and temperature cycle.